1. Field of the Invention
The present invention relates generally to an apparatus for producing a semiconductor single-crystal silicon grown by the floating-zone (FZ) method in which a polysilicon rod is converted into a single-crystal ingot by passing a molten zone, formed by heating a part of the polysilicon rod with a high-frequency induction heating coil, from one end of the polysilicon rod to the other. More particularly, it relates to a self-clamping holder which is able to hold the polysilicon rod using the weight of the polysilicon rod.
2. Description of the Prior Art
In the production of a silicon single crystal grown by the FZ method, there has been used an apparatus which comprises a polysilicon rod holder attached to the lower end of a vertically movable upper drive shaft and disposed in a growth chamber, a seed crystal holder attached to the upper end of a vertically movable lower drive shaft and disposed in the growth chamber, and a high-frequency induction heating coil disposed within the growth chamber at an intermediate portion of the growth chamber. A polysilicon rod and a seed crystal are held by the polysilicon rod holder and the seed crystal holder, respectively. Then, one end of the polysilicon rod is melted down by heating with the high-frequency induction heating coil. The molten tip of the polysilicon rod is contacted and fused with the seed crystal, thus seeding the molten zone. The seeded molten zone is passed through the polysilicon rod by simultaneously moving down the polysilicon rod with the seed crystal while rotating the polysilicon rod relative to the high-frequency induction heating coil. As the zone travels the polysilicon rod, single-crystal silicon freezes at its end and grows as an extension of the seed crystal.
As shown in FIG. 3 of the accompanying drawings, the polysilicon rod holder 2 includes a generally cup-shaped adapter 8 made, in general, of stainless steel and secured by a screw 6 to the lower end of the upper drive shaft 4 with its open end facing downward, and a plurality (six to eight, in general) of clamp screws 12 threaded respectively through a plurality of threaded holes 10 formed in a lower portion of an annular side wall of the adapter 8 at equal angular intervals. Each of the clamp screws 12 has a heat-resistant presser plate 14 attached to a front end of the clamp screw 12, so that by tightening the respective clamp screws 12, a polysilicon rod P is gripped by the presser plates 14. Thus, the polysilicon rod P is held by the polysilicon rod holder 2. In general, the heat-resistant presser plates 14 are made of molybdenum which has a higher melting point than silicon.
When used in the production of a silicon single crystal grown by the FZ method, the apparatus having the conventional polysilicon rod holder shown in FIG. 3 has encountered the following problems.
(1) Accidental Detachment of Polysilicon Rod:
At the final stage of the FZ single-crystal growth process, the molten zone comes up to an upper end portion of the polysilicon rod P adjacent to the polysilicon rod holder 2. In this instance, since the temperature of the upper end portion of the polysilicon rod P reaches to 1000.degree. C. or more, the holder 2 as a whole is heated at a high temperature due mainly to radiant heat emitted from the thus heated upper end portion of the polysilicon rod P. It occurs that due to thermal expansion, the clamping forces exerted by the respective clamp screws 12 on the polysilicon rod P become low, thus causing the polysilicon rod P to detach from the holder 2.
(2) Disqualified Silicon Single Crystal:
Due to the use of a plurality of clamp screws 12, it is very difficult to balance or equalize the clamping forces produced by the respective clamp screws. With this difficulty in making a force balance, the polysilicon rod P is likely to be held in such an inaccurate condition that a longitudinal central axis of the polysilicon rod P draws a circle when the polysilicon rod P held by the holder 2 is rotating. With the polysilicon rod thus held inaccurately, the FZ single-crystal growth process is difficult to achieve in a stable and reliable manner. Even if the FZ single-crystal growth process were carried out, the silicon single crystal would not be desirable qualitywise.